Aqueous dispersions containing copolymers of vinyl esters and ethylene are well known for the production of adhesives for use in the wood, paper and packaging industries. For such uses, it is common to polymerize the comonomers in an aqueous medium containing polymerization initiators and water soluble protective colloids, especially polyvinyl alcohols derived by hydrolysis of polyvinyl acetate. The resulting emulsion polymers can then be formulated with either partially or fully hydrolyzed polyvinyl alcohol to obtain adhesive compositions having varying properties.
For example, U.S. Pat. No. 3,827,996 discloses a process for preparing a stabilized aqueous dispersion of a vinyl ester polymer by copolymerizing, in an aqueous medium, vinyl ester and ethylene in the presence of a partially hydrolyzed polyvinyl alcohol derived from polyvinyl acetate, wherein the polyvinyl alcohol has an average vinyl acetate content of about 5 to 7 mole %, consists essentially of polyvinyl alcohol molecules containing no less than 0.5 mole % and no more than 16 mole % of vinyl acetate and is present in an amount of about 2 to 10 wt % based on the weight of vinyl ester. In Example 1, the polyvinyl alcohol is a blend of polyvinyl alcohol having a vinyl acetate content of 0.5 to 1 mole % and polyvinyl alcohol having a vinyl acetate content of 12 to 16 mole %.
U.S. Pat. No. 4,521,561 discloses a vinyl acetate/ethylene copolymer emulsion which comprises an aqueous colloidal dispersion of a copolymer prepared by the emulsion polymerization of vinyl acetate and ethylene in the presence of a polyvinyl alcohol stabilizing system having an 8 to 10 mole % residual vinyl acetate content, especially comprising a mixture of a polyvinyl alcohol having a 3 to 6 mole % vinyl acetate content and a polyvinyl alcohol having a 10 to 14 mole % vinyl acetate content in a ratio that yields an average residual vinyl acetate content of 8 to 10 mole %. When blended with a partially- or fully-hydrolyzed polyvinyl alcohol, these emulsions are said to provide adhesive compositions with enhanced plasticizer thickening.
U.S. Pat. No. 5,143,966 discloses a vinyl acetate-ethylene copolymer emulsion prepared by the aqueous emulsion copolymerization of vinyl acetate and ethylene in the presence of a polyvinyl alcohol stabilizing system consisting essentially of: (a) 0.2 to 1.5 wt % low molecular weight, partially (85-90 mole %) hydrolyzed polyvinyl alcohol, (b) 0.5 to 2.5 wt % medium molecular weight, partially (85-90 mole %) hydrolyzed polyvinyl alcohol, and (c) 1 to 4 wt % medium molecular weight and/or high molecular weight, fully (98-99+ mole %) hydrolyzed polyvinyl alcohol, the weight percentages based on vinyl acetate monomer.
U.S. Pat, No. 5,633,334 discloses a process for the preparation of emulsion polymers comprising 70 to 95% by weight of a vinyl ester and 5 to 30% by weight ethylene, the polymerization being carried out in the presence of 2 to 8% by weight of a protective colloid consisting of a blend of 1 to 3 percent by weight of 80% hydrolyzed polyvinyl alcohol, 0 to 2 percent by weight of 88% hydrolyzed polyvinyl alcohol and 1 to 3 percent by weight of 96% hydrolyzed polyvinyl alcohol, the blend having an average level of hydrolysis of 85 to 88%. According to the data presented in the Examples, with higher levels of the 96% hydrolyzed polyvinyl alcohol (3.5 to 4 pts) and lower levels of the 80% hydrolyzed polyvinyl alcohol (1 to 1.5 pts), the plasticizer response of the emulsion decreases.
As discussed in certain of the references described above, one important property of a stabilized aqueous dispersion of a vinyl ester/ethylene copolymer for use in an adhesive formulation is its plasticizer response or the increase in the viscosity of the dispersion when mixed with a given amount of plasticizer, such as diisobutyl phthalate (DiBP). This property is generally specified as the Viscosity Increasing Ratio (VIR) and is determined by dividing the viscosity of the dispersion after addition of a specific amount (5, 10 or 15 wt %) of DiBP by the viscosity of the neat dispersion. The property is important since in practice formulators dilute the copolymer dispersion with water and add a plasticizer to produce an adhesive of the desired viscosity so that dispersions with a high VIR reduce the amount of costly and potentially toxic plasticizer required.
According to the present invention, it has now been found that a stabilized aqueous dispersion of a vinyl ester/ethylene copolymer with a high initial viscosity and a high VIR can be produced by conducting the polymerization in the presence of a polyvinyl alcohol stabilizing system consisting essentially of about 0.5 to about 3 wt % of fully (98-99+ mole %) hydrolyzed polyvinyl alcohol and about 1 to about 4 wt % of partially (85-90 mole %) hydrolyzed polyvinyl alcohol having a medium molecular weight and/or high molecular weight, the percentages being based on the total weight of monomers present in the copolymer. Desirably, the dispersion has a low formaldehyde content and a large particle size.